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Table of Contents





This page refers to the following Laird Technologies part numbers:


Part Number 


BTM510 Bluetooth Multimedia Module with external antenna
BTM511 Bluetooth Multimedia Module with integrated antenna
DVK - BTM510 Development Board with BTM510 soldered in place
DVK - BTM511 Development Board with BTM511 soldered in place


The BTM51x module family, with associated development kits, has been available as a mass production part since late 2010. As part of Laird’s commitment to continuous improvement and technical innovation, the product family has been consistently enhanced to provide key benefits for the global customer base. This document provides details and analysis of the enhanced feature set of the BTM51x series of Bluetooth Class 2 Audio modules from Laird Technologies.



The BTM51x series consist of Class 2 Bluetooth modules designed to add robust audio and voice capabilities to OEM products. The modules, which are based on the market-leading Cambridge Silicon Radio (CSR) BC05 chipset, provide an integrated stereo codec and a 64-MIPS Kalimba DSP. Using additional soft codecs, designers can add features such as echo cancellation, noise reduction, and audio enhancement. The module range supports Bluetooth specification v2.1 with Simple Secure Pairing for improved security and easier user functionality.

The BTM51x series offers significant time and cost advantages over an integrated or on-board solution. The modules carry a broad range of certifications -- Bluetooth END product, FCC modular, IC and CE – and support an extensive Bluetooth profile suite including support for SPP, HFP, HSP, DUN, A2DP and AVRCP. Support for a comprehensive AT command interface speeds integration and simplifies application development in the potentially complex world of mono and stereo audio products. A low-cost, feature-rich development kit further speeds customer’s time to market.

The following table summarizes the advantages of BTM51x series module over a Bluetooth chip:



BTM51x Series Module 


Device Life Cycle
  • Because the module is complete, full-featured component, time-to-market is much shorter
  • Because a chip requires more planning for integration onto the target circuit board, time-to-market is longer
RF Design and Integration
  • Requires less integration effort. You can rely on Laird's expertise for the BT and RF design
  • Functions with simple and easy-to-use AT commands
  • Integration onto the host device's board is more complicated; you must have your own RF design experts
  • Laird will provide on-going support and troubleshooting for the module
  • Once you integrate a chip onto a PCB you own the support responsibility
  • Involved the purchase of a single component
  • Involves the purchase of multiple components which may increase cost
Fully Approved
  • Fully approved from a Bluetooth SIG perspective and maintains a wide range of international RF approvals
  • Leveraging these approvals greatly reduces your testing time and compliance cost
  • Requires its own compliance testing for both Bluetooth and RF which increases time-to-market and cost






The BTM51x series has been designed into a wide range of OEM products that require a reliable Bluetooth wireless link for high-quality audio products. Two types of applications for which BTM51x series modules are a good fit are stereo streaming applications and mono voice applications. Stereo streaming applications include smartphone speaker docks, wireless hi-fi audio systems, automotive multimedia devices, and stereo headsets. Mono voice applications include VoIP phones and gateways, intercom systems, vehicle hands-free kits, first responder audio intercoms, and industrial audible alarm systems.


Profile Features / Enhancements






The latest production release of firmware for the BTM51x series (firmware version incorporates an upgrade to the latest ADK2.0 from CSR for the BC05 MM chipset. Laird’s implementation of the ADK 2.0 in our AT command set enables an upgrade of all chosen A2DP encoders and decoders. This means that customers’ audio devices can act not only as the audio receiving role (A2DP sink) but also as the audio transmitter (A2DP source) providing the audio stream for Bluetooth wireless transmission.



This benefit is further enhanced by built-in support for the apt-X™ audio codec for wireline-quality stereo audio. This audio compression solution provides for the highest quality audio with very low latency for audio streaming devices. Support is available at no additional charge. The BTM5 series also supports Bluetooth’s standard SBC (SubBand Codec) codec and the AAC (Advanced Audio Codec), which is Apple’s preferred codec for iOS devices. (AAC technology is IP owned by numerous third parties and requires an independent license for use in the finished OEM product. For details please visit http://www.vialicensing.com.)

In summary, the BTM51x series supports the following A2DP encoders and decoders:

  • Encoders (A2DP source role):SBC, APTX
  • Decoders (A2DP sink role):SBC, APTX, AAC

Soft codec support in standard production firmware enables customers to seamlessly support the vast majority of codec implementations found in today’s electronic devices.







The comprehensive AT command structure implemented on the BTM5 series allows customers the flexibility to support multiple Bluetooth profiles simultaneously. Laird’s years of experience in embedded Bluetooth applications have helped to illuminate specific customer use case scenarios that can only be realized by supporting simultaneous device profiles.


Usage Example

A customer implementing a Bluetooth wireless solution for first responders needed the ability to have a voice communications to BT-enabled gateway cellular device, as well as a data stream for sensor data. The BTM5 series AT command set handles this effortlessly by supporting Headset Profile (HSP) for the mono voice telephone and Serial Port Profile (SPP for the data channel). Similarly a customer required the ability to stream stereo audio for playing music files from their device and a data stream for GPS data – implemented by simultaneously using A2DP and SPP.

Flow Diagrams 1-5 in the Role Switch section outline the various scenarios that are easily addressed within the module’s firmware.


BTM51x F-B Diagram1-3 


BTM51x F-B Diagram4-5 





The latest production release (v18.1.4.0) allows more transparency by providing detailed status information for all connected profiles. For all existing connections, the following items can be queried with a single ATI command:

  • Bluetooth device addresses (BdAddr)
  • Receiver strength indicator (RSSI)
  • The local HCI role (master/slave)
  • EIR support (Enhanced Inquiry Response)



Better control on Bluetooth links is achieved by new AT commands, allowing to request an HCI role change (master/slave) either manually or automatically on connect. This helps to improve Bluetooth performance in particular use cases, e.g. when an A2DP link is initiated from BTM5 to a phone.

Note: Some combinations of the HSP / HFP configuration may be complex due to the actual role specified. For example, it has been identified that simultaneously enabling the Gateway plus Headset / Hands-Free role resulted in an unresponsive system. Please contact your local Laird FAE for further information.



Audio Enhancements



In order to further improve the audio quality supported by the BTM5 series, Laird Technologies has included “Clear Voice Capture” (CVC) v5.0 in the module firmware images since v18.1.2.3.

CVC is a technology provided by Cambridge Silicon Radio (CSR) which utilises the BC05 internal DSP in order to improve the audio quality of SCO links (speech). This includes sophisticated noise reduction and echo cancellation. CVC works with both single and dual-microphone headsets, giving advanced audio enhancements and noise suppression to the near and far end as well as providing packet loss and bit error concealment.


The BTM5 series provides for three different usage scenarios -- Headset with one microphone, Headset with two microphones, and Hands-Free with one microphone -- with a separate DSP image for each, configurable by AT command. Whenever a SCO connection is established, the selected image is loaded on the DSP before the audio stream is connected.

This is where the support of AT commands for CVC ends. From the beginning of a SCO connection with CVC, the customer will use CSR tools, namely “Universal Front End (UFE)/ Universal Parameter Manager” to parameterise and tune the CVC algorithms. UFE is a powerful GUI that allows to hear new parameters immediately as they are written to the DSP for easier tuning. CSR provide documents which describe not only the details of how to tune their CVC algorithms, but also what needs to be taken into account when designing a headset (e.g. mechanics, position of microphones etc.) At the end of the tuning phase, a *.psr file will be created by UFE, containing all DSP parameters.

Once the customer has finished the tuning phase, the DSP parameters need to be written to the BTM51x module in the customer’s production environment. For this purpose, Laird Technologies provide a tool called “PsUpdate.bat” which will modify PSKEYS (DSP parameters) over UART. Using this tool guarantees the Bluetooth SIG approval remains valid.

In addition, a license key is required for the CVC image for production. This license key must be ordered from CSR approved distributors directly and written to the BTM5xx in the customer’s production environment using “PsUpdate.bat”. The license keys vary in price versus volume; one example, Digikey - part # BCSW-CVC-HS-1M-FX, has one of the keys as $0.55 per key for a MOQ on 2000 keys.

The Laird AT command set provides support commands for managing CVC license keys. This includes a check for a valid license key at boot time and when a SCO connection is entered. If a CVC image is loaded and the security check has failed, ERROR 83 is returned.

In order to further assist customers with CVC integration, Laird Technologies has produced a detailed Application Note to cover all aspects of the process. In order to access this information, which includes CSR specific information and IP, a Laird technologies NDA needs to be completed – please contact your local Laird Sales representative to expedite this process.

The diagram below shows a pictorial example of the CVC Development process and Appendix A covers an example of simple AT commands required to set up the enhanced audio link.


BTM51x F-B CVC Dev Phases 




The Music Manager is a Windows based application that allows you to configure and monitor A2DP post processing algorithms. The Music Manager is available with the appropriate version of a Headset Application Development Kit (ADK). It is important to use the version of Music Manager which corresponds to the DSP images contained in a BTM51x firmware release. ATI38 helps identifying the image versions:


BTM51x F-B CVC Ex Setup 


The string “ADK2.0” indicates that Music Manager which comes with CSR’s ADK2.0 is the appropriate version to configure DSP post processing for each of the 3 A2DP decoders contained in this BTM51x firmware release. These decoders provide a collection of audio effects to enhance or fine-tune audio output.

Decoder Types:

  • SBC Decoder
  • AAC-LC Decoder
  • APTX Classic Bluetooth Decoder (security key required)

Audio Effects: 

  • Speaker Parametric Equalisation2
  • Bass Boost3
  • User Parametric Equalisation
  • Stereo 3D Enhancement
  • Compander
  • Post Mastering (Noise Shaped Dither)

Audio effects are provided for both 44.1 kHz and 48 kHz sample rates, and are supported for all the specified decoders listed above. The audio effects Stereo 3D Enhancement, and Dither algorithms are sample rate agnostic. However separate 44.1 kHz and 48 kHz sample rate tuning is required for the Speaker and User Parametric Equalisers, Bass Boost and the Compander.



Interface Support

The BTM51x series is designed from the ground up to enable the widest array of audio applications for OEMs requiring mono and stereo audio capabilities. In order to provide a broad range of interface options, Laird provides for the following hardware interfaces on-board the module:


  • Full 8-pin UART interface at 3.3V logic level for command and data transfer
  • Four (4) signal lines (lines shared with I2S)
  • Four (4) signal lines (lines shared with PCM)
  • Enable use of external codecs or wave file player/recorder
  • Select I2S master or slave role
  • Enter Test mode: I2S cross loopback (internal codec), bidirectional audio
  • If I2S is selected for A2DP sink, 44.1kHz and 48kHz are advertised as supported sampling rates in the SDP record (fixed). Hence, the external codec must support both rates.
  • 4 x general purpose input/output support:
    • 4 freely usable GPIOs available
    • Function mapping (e.g. volume up/down, headset button, etc.)
    • AVRCP operation id mapping (e.g. Play/Stop/Pause, etc.)
  • 2 x LED output (NOTE: See the Automation section)
    • AT commands for on/off/blink/pulse
    • Automatic status indication (connectable/discoverable/connected/audio on) with a range of pre-defined blink patterns as of firmware version





The Automation features implemented into the latest v18.1.4.0 firmware provide OEMs with increased flexibility and scope for their end product implementation. These features allow the OEM to provide enhanced levels of functionality to their end users allowing for seamless, intuitive audio connectivity to smartphones and audio devices. Additionally the integration of higher level Bluetooth functionality into the BTM51x series modules reduces the processing burden for the OEMs host architecture, potentially reducing the overall BOM costs.


  • Host-less operation by push buttons, no external microcontroller required, for typical stereo headset/speaker use cases
  • Auto connect records allow configuration of automatic connections as soon as specified devices enter receiving range
  • Status indication by LED blink patterns or GPIO outputs proving simple methodology to allow end users to determine Bluetooth connectivity status of the host device




In order to speed OEM customers time to market, the BTM5 series of modules is fully approved from a Bluetooth SIG perspective and comes with a wide range of country RF approvals. This enables OEM customers to utilise the module’s approvals within their host product and significantly reduces the approvals/compliance costs – and overall risks – of launching a new product in mass production.

The Bluetooth SIG approval held by the BTM5 series (B016227) is called ‘End Product Listing’ (EPL).This means that the module does not require any additional testing or approvals from a global Bluetooth perspective and allows the OEM to fully market their device using the Bluetooth name and logos. The only obligations for the OEM, as specified by the Bluetooth SIG, are that they apply free of charge to become a Bluetooth Adopter at the Bluetooth SIG and list their final end device on the Bluetooth SIG EPL listing. A handy guide to the Bluetooth SIG process can be found at the end of the detailed User Manual for the BTM51x.

OEMs can take advantage of the modules’ following Regulatory Approvals:

  • FCC Modular (USA): PI4510B (part # BTM510) and PI4511B (part # BTM511)
  • CE (Europe): Declaration of Conformity incorporated into BTM51x User Manual 
  • MIC (Japan): R 201-125696 



Additional Services


The BTM5 series of innovative Bluetooth modules is built utilising over 10 years’ experience designing with CSR silicon. The design of the hardware and firmware implementation is 100% developed by Laird Technologies, allowing flexibility in our approach to firmware customisation.

The structured AT command set covers the vast majority of individual OEMs requirements for allowing access to the myriad functions of the BTM5 module. However, from time to time, individual customers have a particular application requirement or use case scenario that needs a specific function.

Laird maintains an open approach towards new feature requests from customers and assesses each against the backdrop of our own firmware development schedules, multiple requests for similar functions and commercial viability. In the main, these new features can be rolled into the firmware development roadmap and provided to customers as part of the standard production release firmware. If the request is uniquely customer orientated then Laird can offer a custom part number with tailored firmware developed to provide the required functionality against either a NRE firmware development fee or amortised against a volume purchase agreement.

An example of this is providing integration of customer DSP code into the BTM5 series firmware image, such as in an A2DP sink by inserting a custom signal processing block behind the A2DP decoder. Customers can start developing their DSP application using “MyFirstKalimbaDspApplication” / “MySecondKalimbaDspApplication” which is provided within CSR’s BC05 SDK / ADK.



Laird is committed to providing a robust and reliable Bluetooth development experience for all customers, large or small. Each BTM5 series module and development kit is backed by a comprehensive User Manual, Quick Start Guides, product videos and most importantly by regional technical support through our global Field Application Engineer (FAE network). The FAE team is represented by support teams in Asia, Europe and the US who will assist customers through the complete product development cycle – initial evaluation, prototypes, pre-production and mass production roll out.

Examples of the supporting documentation can be found in Appendix B with web links to the content.



Appendix A – AT Command Example For CVC Functionality


The following AT command sequence needs to be sent to the BTM5xx in order to prepare it for CVC – headset 1 mic:


AT Command (HF) 


AT&F* Restore factory default settings
ATS102=$10 Enable Bluetooth Hands-free profile -> Hands-free role. This does not mean that the CVC hands-free image must be used; it also works with CVC headset 1 mic image and CVC headset 2 mic image. HFP allows creation and closure of a SCO channel at any time from either end, therefore it is the most suitable BT profile for the CVC development phase.
ATS512=4 Make module permanently discoverable and connectable
ATS318=1 Select CVC image ‘headset 1 microphone’.
AT&W Store settings
ATZ Reset module. New settings take effect upon reset.
ATI4 Query module’s Bluetooth device address. In this example the response should be 00025b00ff01. Will be required to initiate a connection to this device.



For the CVC image to be loaded, a SCO link must be created. One option is to use a mobile phone as the audio gateway for HFP.

A better option (at least for the development phase) is a second BTM5xx, e.g. a BTM51x development kit, because this allows full control over the SCO link (mobile phones often close the SCO link if no audio is being transmitted). This setup is outlined below.

The second BTM5xx is configured as follows:


AT Command (HFG) 


AT&F* Restore factory default settings
ATS102=$40 Enable Bluetooth Hands-free profile -> audio gateway role
ATS512=4 Make module permanently discoverable and connectable
AT&W Store settings
ATZ Reset module, new settings aply from now onwards
ATI4 Query module's Bluetooth device address. Will be required if we are going to initiate a connection to this device



Currently, CVC is not supported for the audio gateway role of neither HFP nor HSP. In most use cases, the audio gateway will typically be a mobile phone.

Now we can initiate the HFP link. The link can be initiated from either end, in the first table the link is intiated from the HF device (BTM5xx with S318=1) :


AT Command (HR) 


AT+HFD Initiate HFP service level connection (SLC), is what ATI4 returns on the HFG. At the first connection, a successful pairing should occur (“PAIR 0…”). Then we should see many HF indicators (“HFI…”) and the “CONNECT...” message
  • FS8000,INT - sampling rate = 8000 Hz, internal codec
  • CVCID B00A - CVC image with SYSID = 0xB00A loaded
  • HF"AU1" - HF instance audio connection established
AT+HFR Release the SCO link
AT+HRH Release HFP Service level connection (SLC), If SCO link exists it will be released automatically
ATH* Close all connections of the module. This command can be used alternatively to AT+HFH




The second table lists AT commands for initiating the link from the HFG side (BTM5xx)


AT Command (HFG) 


AT+HFGD Initiate HFP service level connection (SLC), is what ATI4 returns on the HF
AT+HFGA >Initiate audio (SCO) link. Response should be:
  • FS8000,INT - sampling rate = 8000 Hz, internal codec
  • HF"AU1" - HF instance audio connection established
AT+HFGR Release the SCO link
AT+HFGH Release HFP Service level connection (SLC), If SCO link exists it will be released automatically
ATH* Close all connections of the module. This command can be used alternatively to AT+HFH



Appendix B - Examples of the Current BTM51x Series Documentation


BTM510 / 511 -Bluetooth Audio Modules


BTM510  BTM511 

BTM510 Module 

BTM511 Module 


The BTM51x series modules are feature rich, BC05 Class 2 modules for high quality audio and stereo Bluetooth applications.


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© 2013 Laird Technologies

This document is issued by Laird Technologies Limited in confidence, and is not to be reproduced in whole or in part without the prior written permission of Laird Technologies. The information contained herein is the property of Laird Technologies and is to be used only for the purpose for which it is submitted and is not to be released in whole or in part without the prior written permission of Laird Technologies.